Ion implantation systems usually include an ion source for converting a dopant material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam may be distributed over the target, typically a semiconductor wafer, by beam scanning, by target movement or by a combination of beam scanning and target movement. A number of different ion implanter architectures are known to those skilled in the art.
The ion implanter typically includes one or more magnets, depending on the beam transport architecture. The magnets perform functions such as mass analysis to remove undesired species from the ion beam and angle correction, or collimation, of the ion beam. Such magnets typically include polepieces on opposite sides of a flight tube through which the ion beam is transported. The magnet produces a magnetic field which deflects charged particles in the ion beam in a desired manner. Examples of ion implanter architectures utilizing magnets are disclosed in U.S. Pat. No. 4,922,106, issued May 1, 1990 to Berrian et al.; U.S. Pat. No. 5,350,926, issued Sep. 27, 1994 to White et al.; and U.S. Pat. No. 6,313,475, issued Nov. 6, 2001 to Renau et al.
Current semiconductor fabrication processes often require high-current, low-energy ion beams. High currents are required to limit implant time, while low energies are required to produce shallow junction semiconductor devices. Low-energy, high-current ion beams are very difficult to transport over large distances due to space charge blowup of the beam. It is known that space charge neutralization of an ion beam containing electrons is necessary for transport of low-energy, high-current ion beams. Electrons can be produced by both beam/surface and beam/gas collisions. However, at low ion beam energies, the cross-section for beam/gas collisions drops off dramatically, so that this method is unsatisfactory. In regions of magnetic field, the electrons that are produced by collisions are inhibited from moving to electron-deficient regions. Therefore, neutralization in regions of high magnetic field is particularly difficult. Techniques for beam containment in magnets are disclosed in U.S. Pat. No. 6,414,329, issued Jul. 2, 2002 to Benveniste et al; U.S. Pat. No. 6,762,423, issued Jul. 13, 2004 to Liebert et al.; and U.S. Pat. No. 6,515,408, issued Feb. 4, 2003 to England et al.
Prior art techniques have had one or more drawbacks, including but not limited to a relatively short lifetime and contamination of the semiconductor wafer. In those configurations that utilize filaments located between the polepieces of a magnet, the space occupied by the filaments reduces the space available for transport of the ion beam. Accordingly, there is a need for improved methods and apparatus for electron injection in magnets.